A conventional plate with a lens array is shown in FIG. 5. Reference may be had to commonly owned U.S. Pat. No. 4,952,037 issued on Ser. No. 07/317,079 filed Feb. 28, 1989 as a continuation of Ser. No. 119,249 filed Nov. 2, 1987 and a continuation in turn of Ser. No. 06/836,339 filed Mar. 5, 1986. Ser. No. 07/317,079 was divided to Ser. No. 07/398,182 of Aug. 24, 1989 of which a continuation Ser. No. 07/616,734 of Nov. 21, 1990 is copending herewith. As shown in FIG. 5, a conventional plate with lens array is constituted by arranging a large number of small gradient index lenses 12 in a glass plate 11. Each small lens 12 is defined by forming a semi-spherical gradient index region in which a refractive index is gradually decreased outwardly from the center to a portion near the surface of the glass plate 11. Each small lens 12 has a function of a convex lens for diffracting a light ray 13 by this gradient of the refractive index. The method of manufacturing the small lenses 12 will be described below. A metal mask is coated on a glass substrate, and mask openings are formed by photolithography at positions where the small lenses 12 are to be formed. The glass substrate is thus prepared with the metal mask on which a large number of openings are arrayed. The glass substrate is dipped into a molten salt containing ions at a high temperature for a predetermined time, the ions serving to relatively increase a refractive index of glass. In the molten salt is performed ion exchange between ions in the glass substrate and ions in the molten salt by thermal diffusion. As a result, a concentration distribution of ions introduced from the molten salt is formed in the glass substrate, and said semi-spherical gradient index regions are thereby formed in the glass substrate.
The plate with lens array on which the large number of small lenses 12 are arrayed can be used, e.g. for a coupling lens array between an LED array and an optical fiber array. However, since the conventional plate with the lens array is prepared so that all the small lenses 12 have the same focal length, its application is undesirably limited to some extent.